Method of reclaiming selenium and metal base from electrophotographic plates

ABSTRACT

A method for separating a coating, such as of selenium, from the smooth surface of a base material such as a metal electrophotographic plate comprising rapidly reducing the temperature of the coated base metal to far below freezing temperature of water, wetting the coated surface of the refrigerated base metal with a liquid such as water which forms a frozen layer such as ice on the coated side of the base metal, and then applying heat to effect separation of the frozen layer from the base metal with the coating adhered to the separated frozen layer, and wherein the coating material is separated as a solid from the liquid phase of the removed frozen layer.

United States Patent [191 Bixby [111 3,837,815 Sept. 24, 1974 METHOD OF RECLAIMING SELENIUM AND METAL BASE FROM ELECTROPHOTOGRAPHIC PLATES [75] Inventor: William E. Bixby, Deerfield, I11.

[73] Assignee: A. B. Dick Company, Niles, 111.

[22] Filed: Mar. 29, 1973 [21] Appl. No.: 346,051

[52] US. Cl 23/293 R, 134/17, 117/119.4, 117/102 R, 241/23, 423/510 [51] Int. Cl... B02c 23/00, B020 21/00, C0lb 19/00 [58] Field of Search 23/293 R, 293 A, 294; 134/4, 5, 17;24l/17, 18, 23; 117/11914, 3.3,

6, 102 R, 102 M, 102 A; 423/510 Primary Examiner-Norman Yudkoff Assistant ExaminerS. J. Emery [57] ABSTRACT A method for separating a coating, such as of selenium, from the smooth surface of a base material such as a metal electrophotographic plate comprising rapidly reducing the temperature of the coated base metal to far below freezing temperature of water, wetting the coated surface of the refrigerated base metal with a liquid such as water which forms a frozen layer such as ice on the coated side of the base metal, and then applying heat to effect separation of the frozen layer from the base metal with the coating adhered to the separated frozen layer, and wherein the coating material is separated as a solid from the liquid phase of the removed frozen layer,

13 Claims, N0 Drawings METHOD OF RECLAIMING SELENIUM AND METAL BASE FROM ELECTROPHOTOGRAPHIC PLATES This invention is addressed to the removal of worn out photoconductive coatings of selenium from their supporting metal base, such as metal plates or cylinders, in a manner which permits recovery of the selenium and without damage of the base material so that the latter can be re-used without the need for refinishing to provide the smooth surface required for the preparation of a photoconductive cylinder or plate upon re-application of a photoconductive coating of selenium or the like.

While the new and novel concepts will hereinafter be described with reference to the removal and recovery of selenium without disturbing the characteristics of the metal baSe, it'will be apparent that the concepts have utility also in the separation for removal of other coatings from a metal substrate without change in the physical or chemical state of the coating material and without altering or damaging the coated surface of the substrate.

In the manufacture of a suitable plate or cylinder having a photoconductive surface formed of a thin coating of a photoconductive material, such as selenium, it is essential to make use of a metal substrate having an exceptionally smooth surface which is relatively free of defects. If it were otherwise, the thin coating of the photoconductive material would reveal the surface defects as defects in the image that is formed and in the copy that is produced thereof. The need for such fine surface finish on the substrate demands that high grade substrate material be used and that extreme care be taken in the preparation of the surface for the photoconductive coating. These add up to a relatively expensive base material for photoconductive plates and cylinders. Thus, it is desirable to be able to re-use the plate or cylinder after the photoconductive coating has been stripped therefrom, without the need to reprocess the plate or cylinder as by refinishing or the like.

Additionally, the selenium or other material, making up the photoconductive coating, is rather expensive material and it would be most desirable, from the standpoint of economics and supply, to be able to remove the coating from the substrate in a manner which enables substantially complete recovery in a form which enables re-use in the production of fresh photoconductive coatings on suitable substrates.

Various techniques have been employed for the removal and recovery of selenium coatings from metal substrates in used photoconductive plates. In U.S. Pat. No. 2,816,008, the selenium is removed by solution in molten alkali metal hydroxide. However, this technique not only changes the nature and form of the selenium by solution in molten alkali metal hydroxide, but the molten alkali metal hydroxide is an expensive and dangerous medium and it leaves the surface of the substrate in an undesirable condition. Similar procedures which rely upon conversion of the selenium into a form for removal by solution, without regard to the effect on the surface of the base metal, are described in U.S. Pat. No. 2,889,206 and in U.S. Pat. No. 2,378,824.

It is an object of this invention to provide a method for complete removal of a coating from a metal substrate without harmful effect on the surface of the substrate and in a manner which enables easy and substantially complete recovery of the coating material after it has been removed.

More particularly, it is an object of this invention to provide a simple and economical method for the complete removal of the thin coating of selenium from the surface of a metal substrate without disturbing the surface finish of the substrate and without introducing surface defects therein, and in which the selenium can be recovered in a desirable form for re-use.

Briefly described, a complete and clean removal or stripping of the selenium coating from the surface of a metal plate can be achieved by a sequence of steps which comprises:

1. rapidly refrigerating the coated plate to a temperature substantially below freezing temperature, such as to a temperature below 50C and preferably to liquid nitrogen temperature of about l96C;

2. exposing the coated surface of the refrigerated plate to a liquid, such as water, whereby a sheath of frozen liquid or ice forms on the coated surface of the plate; this can be achieved by immersing the refrigerated plate in a body of water or by applying water to the surface as by spraying or flowing a layer of water over the surface to build up the desired ice layer; and

3. heating the refrigerated plate with the ice or frozen liquid layer as by means of water, hot gases, electrical heaters, radiant heaters or the like, whereby the layer of frozen liquid separates from the plate, carrying with it the selenium originally present as a coating on the plate.

This leaves the metal substrate with a surface which is freed of the coating without exposing the surface to materials or conditions which alter the surface characteristics thereof and without subjecting the surface to physical -or chemical attack. As a result, the original smooth surface, free of defects, remains after the selenium coating has been stripped from the plate.

At the same time, the selenium stripped from the surface of the plate is not altered by the sequence of freezing for ice formation and heating for ice layer separation and thus can be recovered simply by separating the solid selenium from the water after the ice has been allowed to re-melt. Thus the selenium remains in its original state for recovery and re-use, even without reformulation in the event that it is to be re-used in forming photoconductive coatings on a suitable base metal.

It is believed that the thermal shock to which the selenium coated plate is exposed during the initial refrigeration step operates to effect separation of the selenium layer from the underlying smooth surface of the base plate, and that the subsequent formation of an ice layer and removal provides the medium by which the separated selenium layer is removed from the plate surface in a medium from which it can be easily recovered.

Instead, it may be that the preferential adhesion of the selenium layer for the ice layer, coupled with the weakened bond between the selenium layer and the smooth surface of the base metal, as a result of thermal shock or differences in contraction upon cooling and expansion upon heating, operates to enable the clean removal of the selenium layer with the ice layer formed on the selenium surface.

Theoretically, the metal base operates as a heat sink which retains the extremely cold temperature after the ice layer has been formed on the surface thereof to supercool the ice layer to the point of embrittlement. Thus, separation of the ice layer with the adhered selenium can be effected in response to application of heat to the side having the ice layer, or to the opposite side or both.

When the heating means, such as water at ambient temperature or above, is applied onto the side having the ice layer, the ice layer breaks up into multiple pieces. If the heat is applied to the surface opposite the ice layer, then the ice layer tends to separate more as a continuous sheet. In actual practice, it is preferred to make use of a water spray onto the ice layer since the applied stream of water will operate to wash the separated pieces from the surface of the base metal.

Having described the basic concepts of this invention, the following example is given by waY of illustration, but not by way of limitation, of the stripping of a selenium layer from a photolithographic plate in accordance with the concepts of this invention.

Use is made of an aluminum drum having a diameter of 233 mm and a length of 348 mm, a wall thickness of 6 /2 mm with a thin selenium coating on the outer surface of the drum in which the surface has a smoothness of 8 micro inches.

The drum is immersed, at atmospheric pressure, in a bath of liquid nitrogen having a temperature of about l96C. The drum is immersed until boiling substantially ceases, indicating that the drum has been cooled to approximately the temperature of the liquid nitrogen. In the illustrated modification, the drum was immersed in the liquid nitrogen for 2 minutes.

Upon removal from the bath of liquid nitrogen, the refrigerated drum is quickly immersed in a bath of cold water for a period of time sufficient to form an ice layer on the surface of the drum but insufficient to effect material elevation in the temperature of the metal portion of the drum. In the illustrated example, the refrigerated drum was immersed in the ice water for about 30 seconds.

Under these conditions, the aluminum drum is still far below freezing temperature and the cold is transmitted from the metal drum to the ice layer to cause supercooling of the ice layer with corresponding embrittlement. It is desirable to hold a drum for a period of time, such as 30 seconds to about minutes, preferably 1 to 2 minutes, to enable the desired super-cooling effect to be achieved.

Immediately thereafter, water at ambient temperature is sprayed onto the inside of the cylindrical drum. Under these conditions, the aluminum drum warms up faster than the ice layer and the shell of ice can be removed from the surface of the drum with the selenium bonded to the ice layer.

Instead, the water can be applied, as by spraying, onto the surface of the ice layer. Under these conditions, the ice layer breaks up into pieces which are washed by the water spray from the surface with the selenium adhered thereto.

The ice with the selenium can be allowed to melt, leaving the selenium material as a solid which can be easily separated from the water by filtration, decantation, centrifugation, or the like liquid-solid separating means.

It will be apparent that instead of using water to form an ice layer, use can be made of other liquid which will be reduced to a frozen state under the temperature conditions existing.

It will also be apparent that the concepts described can be applied to strip other coatings from base materials wherein the coating and the base material are characterized by a marked difference in their thermal coefficients of expansion and contraction whereby, in response to exposure to cold shock and/or heat shock, the coating tends to separate from the base material at their interface. When the coating exhibits a preferential adhesion to the layer of ice or other frozen liquid by comparison to the smooth surface of the base material between which it is sandwiched, the separated coating can be cleanly removed from the base material. The process is enhanced when the base material is characterized by a rapid heat transfer rate.

It will be understood that changes may be made in the details of materials and operation without departing from the spirit of the invention, especially as defined in the following claims.

I claim:

1. In the method for reclaiming the selenium and the smooth surfaced metal base in a selenium coated metal based element, the steps of rapidly reducing the temperature of the element to a temperature well below 0C, wetting the surface of the selenium coating with water for a time sufficient to form an ice layer on the surface of the selenium coating, raising the surface of the ice coated element to an elevated temperature whereby the ice layer separates from the smooth surfaced metal base with selenium adhered to the separated ice.

2. The method as claimed in claim I in which the selenium coated metal based element is rapidly reduced to a temperature below -50C.

3. The method as claimed in claim 1 in which the selenium coated element is rapidly reduced in temperature by immersing the element in a bath of liquid nitrogen,

4. The method as claimed in claim 3 in which the selenium coated element is maintained in the bath until vigorous boiling substantially ceases.

5. The method as claimed in claim 1 in which the refrigerated element is wetted with water at least on the selenium coated surface to form an ice layer on the selenium coated surface.

6. The method as claimed in claim 5 in which the ice layer is formed on the selenium coated surface by immersing the refrigerated element in a body of cold water.

7. The method as claimed in claim 5 in which the ice layer is formed by flowing a stream of cold water onto the selenium coated surface of the element.

8. The method as claimed in claim 5 in which the ice layer is formed by spraying the selenium coated surface with water.

9. The method as claimed in claim 1 in which the refrigerated element is wetted with the water for a time sufficient to form the ice layer but insufficient to effect material increase in the temperature of the metal base whereby the metal base remains at a temperature considerably below freezing.

10. The method as claimed in claim 9 which includes the step of holding the ice coated metal base for a period of time sufficient to enable cold to transfer from the cold metal base to the ice layer to super-cool the ice layer.

11. The method as claimed in claim 1 in which the temperature is raised by wetting the selenium coated side of the element with water above freezing temperature.

12. The method as claimed in claim 1 in which the temperature is raised by wetting the side of the element 13. The method as claimed in claim 1 which includes the step of allowing the separated ice layer to melt to the liquid phase having the separated selenium present as a solid therein, and then separating the solid seleopposite the selenium coating with water above freez- 5 nium from the water.

ing temperature. 

2. The method as claimed in claim 1 in which the selenium coated metal based element is rapidly reduced to a temperature below -50*C.
 3. The method as claimed in claim 1 in which the selenium coated element is rapidly reduced in temperature by immersing the element in a bath of liquid nitrogen.
 4. The method as claimed in claim 3 in which the selenium coated element is maintained in the bath until vigorous boiling substantially ceases.
 5. The method as claimed in claim 1 in which the refrigerated element is wetted with water at least on the selenium coated surface to form an ice layer on the selenium coated surface.
 6. The method as claimed in claim 5 in which the ice layer is formed on the selenium coated surface by immersing the refrigerated element in a body of cold water.
 7. The method as claimed in claim 5 in which the ice layer is formed by flowing a stream of cold water onto the selenium coated surface of the element.
 8. The method as claimed in claim 5 in which the ice layer is formed by spraying the selenium coated surface with water.
 9. The method as claimed in claim 1 in which the refrigerated element is wetted with the water for a time sufficient to form the ice layer but insufficient to effect material increase in the temperature of the metal base whereby the metal base remains at a temperature considerably below freezing.
 10. The method as claimed in claim 9 which includes the step of holding the ice coated metal base for a period of time sufficient to enable cold to transfer from the cold metal base to the ice layer to super-cool the ice layer.
 11. The method as claimed in claim 1 in which the temperature is raised by wetting the selenium coated side of the element with water above freezing temperature.
 12. The method as claimed in claim 1 in which the temperature is raised by wetting the side of the element opposite the selenium coating with water above freezing temperature.
 13. The method as claimed in claim 1 which includes the step of allowing the separated ice layer to melt to the liquid phase having the separated selenium present as a solid therein, and then separating the solid selenium from the water. 